DC have done many things with Steph recently that are stupid, but there is at least logic to it. Stupid logic, but logic. They would not sabotage their two animated shows less than a day before they were supposed to show it due to some campaign against her, especially as Barbara IS Batgirl in Young Justice, not Steph or Cass. At the moment, it is much more likely that Steph will enter the show as Spoiler, which DC shouldn't have too much of a problem with
And I have to say, I really hope Steph joins the team.
EDIT: Also, while Stephanie is in the credits as voiced by Mae Whitman (considering she plays Wonder Girl, I'm not surprised), in the episode itself she doesn't say a word. She will almost certainly have a role at some point, otherwise she would have just been a generic character, but you would only know Stephanie was there if you read the credits. Otherwise, she's just a blond girl who dresses in purple. Which brings even less credence to the theory that DC did this to screw with fans of Steph